Yamaguchi Takahide's research while affiliated with National Institute for Materials Science and other places

... The static characteristics of diamond transistors have been extensively studied [19][20][21][22][23][24], however the dynamic characteristics are yet to be thoroughly investigated. For power circuit applications, switching characteristics are significant for realizing the switching behavior and the corresponding switching losses. ...

... The majority of the successful diamond transistors are built thanks to the 2D hole gas that is present on the hydrogen-terminated diamond surface. These H-terminated Field effect transistors (FETs) show good on-state characteristics as this approach solves the low carrier density at room temperature challenge present in diamond caused by the deep dopant levels available [6][7][8][9]. However, thermal and time stability remain the main challenges to overcome, even if different solutions have already been proposed [10,11]. ...

... In addition, the defects in metal-oxides will also lead to trap-assisted tunneling, which will cause high gate leakage current, thus seriously affecting the hole density at the diamond surface [18]. In recent years, alternative gate dielectric materials have been widely studied to overcome the major flaws of metal-oxides, among which hexagonal boron nitride (hBN) stands out for its excellent physical properties [4,12,19]. ...

... Atomically thin hexagonal boron nitride (h-BN) has drawn tremendous attention for nextgeneration electronics owing to its excellent properties, wide bandgap (~5.9 eV) [1], high electric breakdown strength (~12 MV/cm), high thermal conductivity (~600 Wm -1 K -1 ), excellent thermal/chemical stability, and a surface free of dangling bonds [2][3][4]. Due to these remarkable properties, h-BN has been adopted perfectly as an insulating layer in capacitors [5,6], transistors [7][8][9][10][11], an active layer for resistive switching memristors [12][13][14][15][16], single-photon emitters [17][18][19], and ultra-flat anti-scattering substrate and encapsulation layer [20][21][22][23][24] for van der Waals materials nanodevices. All of these applications propose strict requirements on h-BN quality and demand for large-scale reliable and controllable synthesis methodologies to achieve high quality 2D h-BN films [25]. ...

... At lower carrier densities, the resistance increases with decreasing temperature, which is attributed to localization of the hole carriers. The carrier density n c required to make the holes conductive at low temperature in h-BN-gated diamond FETs is an order of magnitude smaller than that in ionic-liquid-gated diamond FETs in our previous studies 30, 31 . This suggests that the hole gas in the h-BN-gated FETs was of higher quality. ...

... Such non-monotonic resistance features are not at all expected for superconducting systems showing purely field induced Cooper pair breaking but are rather more likely to result from WAL due to the presence of spin-orbit coupling, which has been widely observed in diamond interfaces [15,38,39]. The observation of the WAL effect is significant as it is a hallmark feature of spin-orbit coupling in a 2D system, and considering the confinement as well as charging effects intrinsic to the nanocrystals that compose the film we attribute this observation to the interfacial region between individual grains [shown in figures 1(a)-(c)]. ...

... The saturating temperature dependence of the oscillation amplitude is consistent with that of the resistance, indicating that phase coherence saturation should be crucial to the formation of the anomalous metallic state. For the insulating films, as the link resistance rises beyond R Q = h/4e 2 with decreasing temperature, the inter-island capacitance and self-capacitances of the islands can produce a Coulomb blockade to Cooper pair transport (38). Thus, G osc and the phase coherence length decrease as the Cooper pairs become localized. ...

... In addition, high mobility persists at a low temperature, even for a low carrier density of 2 × 10 12 cm −2 (Fig. 4a,b). This carrier density is lower than those required to maintain high mobility in previous studies 31,32,52 . Furthermore, the carrier density (Fig. 4b) is nearly independent of temperature down to the lowest temperature (4.5 K), indicating that freezing out of the carriers does not occur even at such a low temperature. ...

... To understand the nature of the QPT to the Bose insulating phase, uncovering detailed properties of the insulating phase is of prime importance. A pivotal model system to gain a deeper understanding of the Bose insulating phase is Josephson-junction arrays (JJAs) [11][12][13][14][15][16][17][18][19][20][21][22]. They are artificial quantum many-body systems composed of superconducting islands connected via small Josephson junctions. ...

... A charge order phase is also a correlated electronic state, which appears in the quarter-filling band under inter-site Coulomb interactions, and elicits a variety of phenomena such as negative differential resistance [25][26][27][28][29][30][31][32][33][34], ferroelectricity [35][36][37][38][39][40][41][42][43][44], and the thyristor effect [45]. Organic charge order materials based on bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) have been extensively studied and various characteristics related to the crystal structure, counter anion, and molecular arrangement of the unit cell have been revealed [46][47][48][49][50][51][52][53][54][55][56][57]. ...